Bootstrap is a web framework for designing websites and web applications.
Bootstrap contains HTML and CSS-based design templates for front-end development, including features such as typography, forms, buttons, navigation and other interface components.
These can include customisable drop-down menus, navigation bars, different types of buttons. The idea is that these elements can easily be put together to build a website that is mobile-ready.
Bootstrap is open source and hosted, developed, and maintained on GitHub.